1. Field
The present disclosure relates generally to communication systems, and more particularly, to a method and apparatus for uplink transmission power control and timing in coordinated multipoint transmission schemes.
2. Background
Wireless communication systems are widely deployed to provide various telecommunication services such as telephony, video, data, messaging, and broadcasts. Typical wireless communication systems may employ multiple-access technologies capable of supporting communication with multiple users by sharing available system resources (e.g., bandwidth, transmit power). Examples of such multiple-access technologies include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, orthogonal frequency division multiple access (OFDMA) systems, single-carrier frequency divisional multiple access (SC-FDMA) systems, and time division synchronous code division multiple access (TD-SCDMA) systems.
These multiple access technologies have been adopted in various telecommunication standards to provide a common protocol that enables different wireless devices to communicate on a municipal, national, regional, and even global level. An example of an emerging telecommunication standard is Long Term Evolution (LTE). LTE is a set of enhancements to the Universal Mobile Telecommunications System (UMTS) mobile standard promulgated by Third Generation Partnership Project (3GPP). It is designed to better support mobile broadband Internet access by improving spectral efficiency, lower costs, improve services, make use of new spectrum, and better integrate with other open standards using OFDMA on the downlink (DL), SC-FDMA on the uplink (UL), and multiple-input multiple-output (MIMO) antenna technology. However, as the demand for mobile broadband access continues to increase, there exists a need for further improvements in LTE technology. Preferably, these improvements should be applicable to other multi-access technologies and the telecommunication standards that employ these technologies.
On example of an improvement for LTE technology is the use of Coordinated Multipoint Transmission (CoMP) schemes, where multiple base stations such as eNode Bs (eNBs) coordinate DL transmissions to a UE. By coordinating and combining signals received from the eNBs using multiple antennas on the UE, CoMP, will provide consistent performance and quality in accessing and sharing videos, photos and other high-bandwidth services whether the UE is close to the center of an LTE cell or at its outer edges. Thus, CoMP can significantly increase network capacity and performance.
CoMP may be implemented using a variety of schemes. One implementation of a CoMP scheme is a “joint transmission” scheme, where multiple eNBs transmit the same data meant for a UE. In this example, a joint precoding vector spanning all the antennas of all involved eNBs may be used. In another implementation, referred to as a “distributed MIMO” scheme, eNBs transmit different pieces of data meant for a UE as different MIMO layers. For example, a first MIMO layer is transmitted by one eNB, a second MIMO layer is transmitted by another eNB, and so on. In yet another implementation referred to as “coordinated beamforming”, an eNB transmits to its associated UEs using beams that are chosen to reduce interference to UEs in neighboring cells.
CoMP may exist in homogeneous networks and/or heterogeneous networks. In heterogeneous networks, a mix of low power base station nodes, or cells, such as pico, femto and relay base stations are used in addition to macro base station nodes, or cells, such as eNBs. In CoMP, the low power nodes may be referred to as remote radio heads (RRHs). The connection between the RRHs and the eNBs may be via an X2 backlink, which is subject to some latency and limited bandwidth issues; or a fiber backlink, which provides minimum latency and relatively unlimited bandwidth.
In a heterogeneous network, the RRHs may or may not have the same cell ID as the macro cell. In the former case, where the RRHs and the macro cell share a common ID, the macro cell and RRHs form a super-cell with centralized scheduling. In the latter case, control and data may be served by different cells. However, power control and timing issues exist with both cases. For example, in certain situations, the UE may derive path loss information from the wrong cell. Further, the UE may also derive timing information from an incorrect source.
Consequently, it would be desirable to address the issues described above.